TIOBE goes to Alexa goes down the list and gets the top 25 sites that have a search box on the front page, and will tell you how many results come back from a search(this means odd choices like ebay are in the mix). 15 of the 25 are Google owned ie google.com, google.fr, youtube.com. They then search for "X programming" and use the reported number of results.

Therefore it would be very easy for someone at google to manipulate the index. I am not sure why they would bother since the index means nothing. There are much easier ways for Google to make Dart a thing if they wanted it to be a thing.